It is known that a dispersion-free single mode lightguide fiber can provide a bandwidth in excess of 100 GHz.km at a desired wavelength. Presently, the current effort for further development of such fiber is focused on the reduction of transmission loss so that the repeater span distance may be increased. One of the means for achieving a lower loss is to shift to a longer operating wavelength, typically in the neighborhood of 1.50 .mu.m.
Additionally, efforts have been devoted to the fabrication of single-mode fibers with graded refractive index which provide lower loss and a dispersion-free fiber. The grading of the refractive index in the core is obtained by doping with certain chemicals (e.g., germanium). In particular, single mode fiber with a triangular-index profile have been fabricated with an attenuation loss of 0.24 dB/km and minimum dispersion, both occurring at a wavelength of 1.55 .mu.m.
The random bend losses of single mode fibers in terms of a variety of the configurations of refractive index profiles; convex, parabolic, step and concave shapes have been investigated and are set forth in an article entitled "Random-Bend Loss Evaluation in Single-Mode Optical Fiber With Various Index Profiles" by M. Kubota et al., in Trans. IECE Japan, E63, 723 (1980). This analysis was developed based on the loss mechanism due to the small deviation of the profile caused by bending the fiber. It was found that the bending loss of the convex structure, particularly in a parabolic refractive index profile is lower than that of the step-index profile.
The worst case was found to be the concave shape that is somewhat similar to an index profile possessing a central dip. It is well known that the optical fibers produced by the MCVD process inevitably accompany a central dip (or burnout) in its index profile due to evaporation of a dopant (i.e., germanium) during the preform collapsing stage. When considering the configuration of a triangular-index profile having a central dip one can suspect that the profile may, undesirably, be structurally pliant to an external force and thus easily induce a high bending loss. Also, it is known that single mode fibers having a step index profile have a relatively small core diameter which presents difficulties when splicing fiber ends together.
Accordingly, there is a need to provide a single mode lightguide fiber with minimal bending loss and relatively large diameter cores.